FIELD OF THE INVENTION
The invention relates to a method and an apparatus for determining the neutron flux density of a neutron-emitting source, in particular a reactor core in a nuclear power facility, having a plurality of fuel assemblies.
An apparatus for determining neutron flux density, a so-called neutron measurement system, is often used in a nuclear power facility to monitor start-up and shut-down procedures. The neutron measurement system has neutron detectors which, in particular, supply a signal proportional to the neutron flux density.
The neutron flux density, when a nuclear power facility is in the shut-down, sub-critical state, differs by several orders of magnitude from that when the nuclear power facility is producing power.
A book entitled "Strahlung und Strahlungsme.beta.technik in Kernkraftwerken" Radiation and Radiation Metrology in Nuclear Power Facilities!, published by Elmar Schrufer, Elitera Verlag, Berlin, 1974 deals comprehensively with the construction and method of operation of neutron detectors, particularly in Sections 3.4, 6.1 and 6.2 thereof. As described in Section 6.1, neutron flux measurement systems with neutron detectors are used to determine the reactor power of a nuclear power facility. In that case a neutron detector may be disposed both outside and inside the reactor core, between adjacent fuel assemblies or elements. The neutron detector may be configured in such a way that it can be moved along a major axis. It can thus be removed from the reactor core during regular power operation of the nuclear power facility.
With regard to the method of operation and measurement accuracy, a distinction is drawn between three different systems for neutron detectors. So-called pulsed systems are preferably used in a boiling water nuclear power facility, where the intention is to achieve high sensitivity in strong gamma radiation. Since in that case low pulse rates can be measured much more easily than very small currents, pulsed systems can also be used to measure lower neutron flux densities. The low gamma pulse level allows discrimination between neutrons and the gamma radiation, for example by using threshold value discriminators. The operating range of a pulsed system covers neutron flux densities in a range from about 10.sup.-1 neutrons/(cm.sup.2 .multidot.s) to about 10.sup.5 neutrons/(cm.sup.2 .multidot.s). That corresponds to a reactor power level of up to about 10.sup.-3 %.
So-called direct-current systems are preferably suitable for medium and high neutron flux levels in a range from about 10.sup.2 neutrons/(cm.sup.2 .multidot.s) to about 10.sup.9 neutrons (cm.sup.2 .multidot.s). Discrimination between neutrons and the gamma radiation is preferably carried out using so-called gamma-compensated ionization chambers. At low neutron flux levels, the use of a direct-current system is generally limited by the influence of the gamma radiation. A neutron detector in a direct-current system is preferably based on a fission chamber and/or boron meter, as are described, for example, in Section 3.4 of the above-mentioned book.
In the case of the so-called alternating-current system, the alternating current which is produced in a fission chamber, ionization chamber or boron meter is superimposed on the direct current and is used to form information. Due to the high ionization rate of the gas in such a chamber, it is no longer possible to resolve the individual pulses from the ionized gas separately. In consequence, a direct current is produced as a mean value, and an alternating current is superimposed thereon. The mean square value of the alternating-current signal is directly proportional to the neutron flux density, as is the direct current as well. The ratio of the signal from the detected neutrons to the signal caused by gamma radiation in an alternating-current system may be greater by a factor of 1000 than that in a direct-current system. An alternating-current system is thus preferably suitable for the medium and high ranges of reactor power levels, with neutron flux densities between 10.sup.6 neutrons/(cm.sup.2 .multidot.s) and 10.sup.14 neutrons/(cm.sup.2 .multidot.s) . An alternating-current system is thus also suitable for the power range of a nuclear power facility.